This invention relates to a fill-level detector employing the radar principle for gaging the level of the lower of two substances layered one atop the other within a container, said detector incorporating a first electrical conductor and a second electrical conductor, both extending parallel to each other in an essentially straight direction and protruding into the lower substance; a generator positioned outside the lower and the upper substance at the end of the first electrical conductor and, respectively, of the second electrical conductor for generating and transmitting an electromagnetic signal; and a transducer provided outside the first and the second substance at the end of the first electrical conductor and, respectively, of the second electrical conductor, for detecting a reflected portion of the electromagnetic signal.
Fill-level detectors of the type described above are currently being marketed by Krohne, S-A under such trade names as Reflex-Radar BM 100. The detection process of this type of fill-level gaging device, operating by the radar principle, is based on TDR (time domain reflectometry) measurements, a concept which has been used for instance in cable testing and which resembles that of radar equipment. For example, an extremely short electric pulse in one of these TDR fill-level detectors is guided along two essentially straight electrical conductors into a container holding a substance such as a liquid, a powder or a granular material whose fill level is to be determined. The short electric pulse transmitted into the container via the two electrical conductors is reflected by the surface of the substance and the reflected portion of the short electric pulse is captured by a transducer in the detector system. The reflected portion of the short electric pulse is a function of the relative dielectric constant or permitivity of the substance and increases with the augmentation of the latter. The runtime of the signal is proportional to the distance between the pulse generator, i.e. the transducer, and the surface of the substance in the container. Varying environmental conditions, whether a rising or falling atmospheric pressure or temperature, have no effect on the accuracy of the TDR fill-level detector. Moreover, the runtime of the signal is not influenced by the dielectric constant of the substance whose fill level is to be measured.
Apart from the detection of the fill level of one given substance in a container, however, there are applications which require the determination of the fill level of two substances layered one on top of the other. Such stratification can occur when the substances differ in terms of their intrinsic density. Performing such measurements with a conventional TDR fill-level detector mounted on top of the container is possible without difficulty only if the lower-density substance also has the lower dielectric value, meaning that the substance forming the upper layer has a lower dielectric coefficient than the substance underneath it.
As in the case described further above, the measurement can be obtained in a way similar to that for a regular fill-level determination in that a short electric pulse is generated and guided into the layered substances via the two electrical conductors protruding into them. In the process, a certain portion of the short electric pulse is reflected off the surface of the upper substance while the remaining portion of the short electric pulse penetrates into the upper layer and continues on within the same, with the propagation rate of that residual pulse traveling through the upper layer diminishing as a function of the dielectric coefficient of the upper substance. The portion of the short electric pulse continuing on through the upper layer is then partly reflected at the interface between the upper and the lower substance while a small percentage of the residual pulse penetrates into the lower substance. However, given the high dielectric coefficient of the lower substance, most of the residual pulse that passed through the upper layer is reflected at the interface between the upper and the lower layer, thus allowing that reflected residual pulse to be detected by the transducer. If the dielectric coefficient or constant of the upper substance is known, it is possible to determine the fill level of both the upper and, respectively, the lower substance.
However, in cases where the upper layer is the substance with the higher dielectric coefficient, the portion of the short electric pulse reflected off its surface is typically large enough that the portion of the short electric pulse effectively penetrating into the upper substance and potentially reflected at the interface between the upper and the lower layer is too insignificant for a reliable TDR measurement. Where that is the case, any measurement employing a conventional TDR fill-level detector is possible only if the TDR fill-level detector is mounted not on top of the container but at its bottom. Only then would the short electric pulse xe2x80x9cseexe2x80x9d the substance with the lower dielectric coefficient first, i.e. before it impinges on the substance having the higher dielectric coefficient at whose interface with the lower dielectric coefficient the major portion of the short electric pulse would be reflected. However, mounting a TDR fill-level detector underneath the container is not only structurally complex, if at all possible, but it can also entail serious safety hazards.
It is therefore the objective of this invention to provide a fill-level detector which can be mounted on top of a container, which operates by the radar principle and which permits the gaging of the fill level of the lower of two substances layered in the container one atop the other, even when the upper substance has a lower density but a higher dielectric coefficient than the lower substance.
The fill-level detector according to this invention which solves the problem referred to and described above, is characterized in that the electromagnetic signal can be coupled into the lower substance at the end of the first electrical conductor positioned in the lower layer and that a portion of the electromagnetic signal reflected at the interface between the upper and the lower substance can be detected by the transducer. For two layered substances, the invention thus provides for the electromagnetic signal to be coupled directly into the lower substance and for the portion of the electromagnetic signal that is reflected at the interface between the lower and the upper substance to be detectable, so that, when the dielectric coefficient of the lower substance is known, the fill level of the latter can be determined. The strong reflection of the electromagnetic signal at the point of transition to the upper substance with the high dielectric coefficient is thus utilized for the measurement and the electromagnetic signal, unlike that in conventional TDR fill-level detectors, is not attenuated before it reaches the lower substance.
In a preferred, embodiment according to this invention, the electromagnetic signal emanating from the generator can be coupled into the first electrical conductor and transmitted through that conductor to the end of the latter that is positioned in the lower substance without the signal making contact with the upper and the lower substance. Since in the first electrical conductor the electromagnetic signal is propagated at the speed of light, its runtime in the first electrical conductor can be easily determined so that, when the dielectric coefficient of the lower substance is known, the fill level of the latter can be easily calculated based on the total runtime of the electromagnetic signal and its reflected portion. The TDR fill-level detector according to this invention is preferably further enhanced in that the electromagnetic signal and its portion that is reflected at the interface between the lower and the upper substance can be guided in the lower substance between the two electrical conductors.
For simplifying the coupling of the electromagnetic signal into the first electrical conductor, that first electrical conductor is preferably hollow and ideally in the form of a rigid tube. The fill-level detector according to this invention can preferably be further enhanced in that the first conductor contains an inner conductor which is electrically insulated from the inner surface of the first electrical conductor. It may suffice to provide such insulation by spacing the inner conductor in the first electrical conductor from the inner surface of the latter. Preferably, however, the inner conductor inside the first electrical conductor is provided with an insulating jacket, preferably of PTFE. With particular preference, the inner conductor within the first electrical conductor is so designed that uniform impedance prevails over essentially the entire length of the inner conductor and the first electrical conductor.
In a preferred, embodiment of the TDR fill-level detector according to this invention, the electromagnetic signal can be coupled into the inner conductor at the end of the first electrical conductor situated outside the lower and the upper substance, it can then be decoupled from the inner conductor at the end of the first electrical conductor positioned in the lower substance and transferred into the second electrical conductor, following which it can be guided in the lower substance between the first electrical conductor and the second electrical conductor. At the end of the first electrical conductor positioned in the lower layer, the inner conductor is preferably connected in electrically conductive fashion to the second electrical conductor.
Preferably, for increased structural strength of the TDR fill-level detector according to this invention, at least one horizontal brace is provided between the first electrical conductor and the second electrical conductor. Of course, any such cross brace will normally have to be electrically insulating. However, in the preferred, embodiment of the TDR fill-level detector according to this invention, the brace is provided at the end of the first or, respectively, second electrical conductor positioned in the lower substance and is then utilized as an electrical connection, insulated from the first electrical conductor, between the inner conductor and the second electrical conductor.
Finally, in a preferred embodiment of the TDR fill-level detector according to this invention, the end of the first electrical conductor situated in the lower substance is provided with a seal preferably consisting of PTFE and/or Viton.